The semiconductor industry is moving into the 21st century with accelerating technological speed driven by small feature sizes and large wafers. This advanced technology capability will become increasingly more difficult to harness and more costly to implement. The Semiconductor Industry Association (SIA) Road map projects that the 0.18 micron/300 mm wafer technology generation in 2001 will require a level of 0.01 defects/cm2 to produce high yield IC products. Not only is this density very low (⅔ defects per 300 mm wafer), but particles as small as 0.06 micrometer (approximately 100 atoms) in diameter can cause electrical IC product failures. Low defect levels are critical for economic success in the IC industry. Table 1 illustrates the effect of defect density level on test yield for several 0.18 micron products: A dynamic RAM memory (DRAM) of 1 Gigabits per chip, a 1000 MIP microprocessor, and a system-on-a-chip IC product (SOC). An increase of defect density (microprocessor) from 0.01 D/cm2 to 0.05 D/cm2 reduces the test yield from 70% to 12%. The IC industry's future economic success will have strong dependency on its ability to develop technology of tool systems that maintain very low defect levels, even as the industry produces finer and finer feature sizes. This yield analysis focuses on three products: DRAM, microprocessor, and system on a chip. Test yields were rigorously calculated for these three products and three technology generations—0.35 micron, 0.25 micron, and 0.10 micron. Table 2 illustrates the specific yields utilized in the study. The IC industry needs technology tools that will eradicate defects in order to achieve the very low defect levels required, even as the industry produces finer and finer feature sizes.
Surface contaminant defects include discrete pieces of matter that range in size from submicron dimension to granules visible to observation with the unaided eye. Such contaminants may be fine dust, dirt particles, or unwanted molecules comprised of elements such as carbon, hydrogen, and/or oxygen. Particulate contaminants (“particulates”) frequently adhere to a surface by weak covalent bonds, electrostatic forces, van der Waals forces, hydrogen bonding, coulombic forces, or dipole-dipole interactions, making removal of the particulates difficult. Particulates frequently encountered in practice include polysilicon slivers, photoresist particles, metal oxide particles, metal particles, and slurry residue. It is known that not all particulates are equally undesirable. For example, particulates that adhere at some non-sensitive portions of the IC circuitry may have no effect on operation or performance and need not necessarily be removed (“don't cares”). On the other hand, particulates that adhere at critical locations (“killer defects”) can cause failure of the IC circuitry and must be removed for proper operation. In certain instances, the presence of surface contaminants renders the contaminated substrate less efficient or inoperable for the substrate's designated purpose. In semiconductors, surface defects due to minor molecular contaminants often render semiconductor masks or chips worthless. As shown by Tables 1 and 2 below, reducing the number of molecular surface defects on a semiconductor wafer by even a small amount can radically improve semiconductor chip test yields. Similarly, removing molecular surface contaminants, such as carbon or oxygen, from the surface of silicon wafer circuit layers as deposited on the wafer or between deposition of layers significantly improves the quality of the IC chip produced.
TABLE 1ProductDefect DensityTest Yield1 Gigabit DRAM0.01 Defect/cm281%1 Gigabit0.03 Defect/cm253%DRAM1 Gigabit DRAM0.10 Defect/cm212%Microprocessor0.01 Defect/cm270%(1000 MIP)Microprocessor0.03 Defect/cm228%(1000 MIP)Microprocessor0.05 Defect/cm212%(1000 MIP)System on a0.01 Defect/cm264%Chip (SOC)System on a0.03 Defect/cm225%Chip (SOC)System on a0.04 Defect/cm212%Chip (SOC)
TABLE 2ProductSizeDefect/cm2Maximum Yield64M DRAM0.350.05/cm267%micrometer200 MIP0.350.05/cm242%MicroprocessormicrometerSOC0.350.05/cm227%micrometer256 DRAM0.250.03/cm270%micrometer600 MIP0.250.03/cm246%MicroprocessormicrometerSOC0.250.03/cm235%micrometer1 G DRAM0.180.01/cm281%micrometer1000 MIP0.180.01/cm270%MicroprocessormicrometerSOC0.180.01/cm264%micrometer
The need for clean surfaces, free of even the finest contaminants, has led to the development of a variety of currently used surface cleaning methods. These known methods, however, each have their own serious drawbacks. For example, widely used chemical and mechanical cleaning techniques require the use of cleaning tools and agents that can introduce as many new contaminants to a treatment surface as they remove. Another currently used method for cleaning substrate surfaces without outside agents requires that the treatment surface be melted to release contaminants which are then removed by ultra high vacuum pressure. This method has the disadvantage that the surface being treated must be briefly melted, which may be undesirable, as for example, when a semiconductor surface is cleaned between deposition of circuit layers and it is desired that the integrity of the previously deposited layers not be disturbed. A further disadvantage with this process is that ultra high vacuum equipment is both expensive and time consuming to operate. Annealing treatment methods suffer similar drawbacks. When a surface is cleaned by annealing methods, the treatment surface of the substrate being cleaned is heated to a temperature that is generally below the melting point of the material being treated but high enough to enable rearrangement of the material's crystal structure. The surface being treated is held at this elevated temperature for an extended period during which time the surface molecular structure is rearranged and contaminants are removed by ultra high vacuum. Annealing cleaning methods cannot be used where it is desired to preserve the integrity of the existing structure being cleaned.
Another currently utilized cleaning method, known as ablation, suffers from its own particular drawbacks. With ablation, a surface or contaminants on a surface are heated to the point of vaporization. Depending on the material being ablated, the material may melt before being vaporized, or the material may sublimate directly on heating. With ablation cleaning techniques, if damage to the treatment surface is to be prevented, the ablation energy must be exactly aimed toward contaminants rather than toward the surface on which the contaminants lie, a difficult task when the contaminants are extremely small or randomly spaced. Even where the ablation energy can be successfully directed at a contaminant, it is difficult to vaporize the contaminant without also damaging the underlying surface being treated.
Various other techniques for cleaning semiconductor surfaces and the like have been described in the background art. An article by Bedair et al., “Atomically Clean Surfaces by Pulsed Laser Bombardment,” J. Applied Physics, Vol. 10 No. 12 (Nov. 1969), describes research using low-energy electron diffraction (LEED) to investigate use of high-power Q-spoiled pulsed lasers for cleaning Ni and Si crystal surfaces in vacuum. Some conditions in this research produced irreparable surface damage.
An article by McKinley et al., “Atomically Clean Semiconductor Surfaces Prepared by Laser Radiation,” J. Physics D: Appl. Phys., Vol. 13 (1980), pp. L193-L197, describes preparation of atomically clean surfaces using laser radiation.
The article by Philip E. Ross in Scientific American 1980, vol. 262 No. 6, pp. 86-88, “Dust Busters: Laser Light Submicron Motes from Silicon Wafers,” summarizes some research directed toward developing methods of cleaning particles from semiconductor wafers, including methods of Susan D. Allen, methods of Werner Zapka and Andrew C. Tam, and methods of Robert J. Baseman and Douglas W. Cooper.
In IBM Technical Disclosure Bulletin, December 1982, pp. 3775-3776, an article, “Laser Activated Cleaning and Etching System and Method,” describes a system which provides a laser-activated gas which reacts with the surface of a sample to clean the surface or to thin the surface. The gas reacts locally with the surface in the regions irradiated by the laser. For example, gaseous SF6 reacts with Si in response to a focussed argon ion laser irradiating a Si surface.
An article by K. Imen, S. J. Lee, and S. D. Allen, “Laser-assisted Micron Scale Particle Removal,” Applied Physics Letters , Vol. 58, January 1991, p. 203-205, describes a laser-assisted particle removal technique in which the contaminated substrates were dosed with water, which preferentially adsorbs in the capillary spaces under and around the particles. The substrates were then subsequently irradiated with transverse, electric, atmospheric CO2 laser pulses. At the CO2 laser wavelength, the beam energy is mainly absorbed in the water and not in the substrate.
W. Zapka, W. Ziemlich, and A. C. Tam, “Efficient Laser Removal of 0.2 mm Gold Particles from a Surface,” Applied Physics Letters, Vol. 58 No. 20, p. 2217-2219 (May 1991), describes laser cleaning with pulsed ultraviolet and infrared lasers in which a liquid film of thickness on the order of a micron is deposited on the surface (only at the irradiation location) with a pulsed deposition of liquid just before the pulsed laser irradiation. The liquids tried were water, ethanol, and isopropanol. Zapka et al. recommend that it is generally preferable to choose the incident laser wavelength to be strongly absorbed by the substrate, rather than by the thin water film or by the material of the small particle.
In IBM Technical Disclosure Bulletin, June 1992, pp. 70-71, an article, “Laser Cleaning of a Delicate (Easily Laser Damaged) Surface,” describes a technique involving pulsed deposition of liquid droplets onto the particulates on the surface together with pulsed laser irradiation at large incidence angle (typically larger than 45°). This publication also refers to several other articles related to the background art, including W. Zapka and A. Tam, “Particle Removal from a Surface by Excimer Laser Radiation,” rut published in CLEO 1990 Technical Digest, Series Vol. 7 pp. 227-228 (1990), and in the same publication at pp. 228-229 an article by K. Imen, S. J. Lee and S. D. Allen, “Laser Assisted Micron Scale Particulate Removal.”
In IBM Technical Disclosure Bulletin, Vol. 39 No. 3 (March 1996) pp. 175-176 an article, “Fast Laser Steam Cleaning by Continuous Film Deposition and Pulsed Laser Irradiation of a Moving Surface,” describes an apparatus for laser steam cleaning of surfaces. A part to be cleaned is moved continuously as a thin liquid film approximately one micrometer thick is continuously deposited on its surface using a nozzle, while pulsed laser radiation is applied downstream to superheat the liquid film, producing steam cleaning action.
An article by Tam et al., “Laser Cleaning Techniques for Removal of Surface Particles,” Journal of Appl. Phys., Vol. 71 No. 7, p. 3515 ff., the entire disclosure of which is incorporated herein by reference, describes experiments that employed flash laser heating, using short-pulsed laser irradiation of a surface for effective removal of particulate contaminations of sizes as small as 0.1 micrometer. The pulsed laser radiation was used with or without the simultaneous deposition of a thin liquid film on the surface to be laser cleaned. Table 3, from the article by Tam et al., lists a number of cleaning methods for removing small particles, along with lower limits of particle diameters and the likelihood of destroying sensitive parts. The highest efficiency method found by Tam et al. included choosing a laser wavelength that is strongly absorbed by the surface together with pulse-depositing a water film of thickness on the order of microns onto the surface shortly before the pulsed laser irradiation. This permitted the effective removal of particles smaller than about 20 micrometers, down to as small as 0.1 micrometer, from a solid surface using a modest ultraviolet laser fluence of 0.1 J/cm2.
TABLE 3Cleaning method limitations (Prior art) from Tam et al.Lower limit of diameterLikelihoodof particles removedof destroyingMethod(micrometers)sensitive partsUltrasonic cleaning25yesWiping5yesBrush scrubbing0.5yesHigh-pressure jet spraying0.5yes(4000 psi)Etching0.5High-pressure jet spraying0.3yes(10,000 psi)High-pressure jet spraying0.2yes(15,000 psi)Megasonic cleaning0.2
The semiconductor industry has a continuing need for cleaning methods with improved performance in removing the smallest-diameter particulate contaminants, especially those with diameters of less than about 0.2 micrometer. For particulate contaminants with diameters of more than about 0.2 micrometer, megasonic cleaning is available. Conventional methods are not adequate for the smallest achievable device structures or for the highest desired cleaning standards. Another desirable improvement is in the reduced or eliminated need for deionized water.
Surface cleaning by melting, annealing, and thermal ablation can be conducted with a laser energy source. However, using a laser energy source to remove contaminants from a surface by melting, annealing or thermal ablation does not overcome the inherent disadvantages or these processes. For example, in U.S. Pat. No. 4,292,093, “Method Using Laser Irradiation For the Production of Atomically Clean Crystalline Silicon and Germanium Surface,” the laser annealing method disclosed requires both vacuum conditions and energy levels sufficient to cause rearrangement and melting of the treatment surface. Other known laser surface cleaning methods involving melting or annealing require similar high energy lasing and/or vacuum conditions, as disclosed in U.S. Pat. Nos. 4,181,538 and 4,680,616. The method of U.S. Pat. No. 3,464,534 suffers the same drawbacks as other high energy laser thermal ablation methods.
The method of U.S. Pat. No. 4,980,536 to Asch et al. uses a high power density excimer laser pulse directed to both front and back sides of a mask to remove small particles. The method of U.S. Pat. No. 4,987,286 to Allen uses an energy transfer medium interposed between each particle to be removed and the surface to which the particle is adhered. The method of U.S. Pat. No. 5,151,135 to Magee et al. uses short pulses of ultraviolet laser light of controlled power density for cleaning single-crystalline and amorphous surfaces. The method of U.S. Pat. Nos. 5,283,417 and 5,393,957 to Misawa et al. uses two lasers, a pulsed laser and a trapping laser, to perform modification and processing of particles and microcapsules. The method of U.S. Pat. No. 5,332,879 to Radhakrishnan et al. for removing trace metal contaminants from organic dielectrics such as polyimide uses pulsed ultraviolet radiation to remove the contaminants by a process of ablation. The method of U.S. Pat. No. 5,637,245 to Shelton et al. uses a laser for cleaning equipment surfaces and provides a barrier layer at the surface to be cleaned. The barrier layer ensures that energy from the laser light is evenly distributed and shields the surface from oxygen to prevent oxidation of the surface.
FIGS. 8 and 9 show a comparison of test yield learning for two cases in fabricating a hypothetical 1000 MHZ semiconductor product having 30 mask levels, with 0.1 8-micrometer minimum critical dimensions. The lower curve 100 of FIG. 8 shows a barrier at about 28% limited yield with 0.03 defects/cm2 for water cleaning. The higher curve 110 of FIG. 9 shows the enabling character of laser cleaning, with 73% yield and in with 0.008 defects/cm2.